This invention relates to semiconductor devices and, more particularly, to steps useful in the manufacture of semiconductor devices by temperature gradient zone melting.
The temperature gradient zone melting process is carried out by selectively depositing an impurity on a body of semiconductor material and exposing the body to a temperature gradient while the entire body is maintained at a preselected elevated temperature. Typically, if the temperature of the body is in excess of about 800.degree. or 900.degree. C., a temperature gradient of a few degrees to a few hundred degrees will cause the impurity to migrate through the body, from cold to hot, leaving in its path a recrystallized region doped to the solid solubility limit of the impurity in the semiconductor wafer at the migration temperature. As a specific example, migration of aluminum through a semiconductor wafer approximately 16 mils thick at 1200.degree. C. with a gradient of about 5.degree. across the wafer will take place in 5 to 10 minutes.
For further background information relating to temperature gradient zone melting, reference is made to U.S. Pat. No. 3,899,362 issued to H. E. Cline et al and U.S. patent application Ser. No. 578,807 filed May 19, 1975, in the name of John K. Boah and entitled, "Temperature Gradient Zone Melting Utilizing Infrared Radiation," both assigned to the present assignee.
A problem occasionally encountered during the manufacture of semiconductor devices by temperature gradient zone melting is that the body of impurity material sometimes breaks up during migration. This gives rise to two distinct effects. The first effect can occur when forming large area regions by migrating a wire through a semiconductor wafer. If the wire breaks into two shorter sections, a nondoped region will be formed and the desired large planar region will actually be two smaller continuous planar regions.
The second effect occurs when small particles of the impurity material become separated from the main body thereof during migration. Smaller bodies of material migrate more slowly and thus may still be disposed inside the semiconductor wafer at the termination of migration. If a particle of the metallic impurity remains at or near a P/N junction, it can short the junction or substantially increase the junction leakage current.
These problems do not generally occur, but when they do, device yield suffers. Semiconductor manufacturing is an extremely competitive, cost-conscious industry, and thus, higher yields are constantly being sought.
It is an object of this invention, therefore, to provide a process that is compatible with temperature gradient zone melting procedures and alleviates the aforementioned problems.